Transmission line for pulse forming networks



Jan. 28, 1958 TRANSMISSION LINE FOR PULSE FORMING NETWORKS Filed Nov. 3, 1952 2 Sheets-Sheet 1 TRIGGER SWITCH Z Z SOURCE TUBE I 2 T T T I RICHARD M. WHITEHORN INVENTOR.

W, izmeaym ATTORNE S R. M. WHITEHOIRN 2,821,685

Jan. 28,1958 I R. M. WHITEHORN 2,

TRANSMISSION LINE FORPULSE FORMING NETWORKS 7 Filed Nov. 3, 1952 v 2 Sheets-Sheet 2 RICHARD M. WHITEHORN INVENTOR- ATTORNE S United States Patent TRANSMISSION LINE FOR PULSE FORMING NETWURKS Richard M. Whitehorn, Concord, Mass, assignor to Bendix Aviation Corporation, Towson, Md., a corporation of Delaware Application November 3, 1952, Serial No. 318,416

2 Claims. (Cl. 333-31) This invention relates to transmission lines and more particularly to transmission lines for use in networks for generating impulses of very short duration and high intensity.

Such impulses are useful in impulse transmission systems and are particularly well adapted to the needs of radio pulse echo systems in which high frequency pulses of great intensity and extremely short duration are periodically generated in a device such as a magnetron. The envelope of these trains is required to be of rectangular form and this form may be obtained by exciting the magnetron by rectangular impulses of corresponding form and intensity.

The problem of generating direct current impulses of the required duration, shape and intensity has been severe since it is diflicult to obtain direct currents of the required voltage and pulse transformers capable of handling the desired waveform without impairing its shape are expensive, bulky and difiicult to make.

One solution of this problem has employed a pulse forming network consisting of a plurality of cascaded transmission line sections having different characteristic impedances. In the use of the network a charging voltage is applied to the input terminals and these terminals are subsequently shorted to discharge the network. As the network discharges successive reflections at the junctions of the sections cause the discharge to take the form of a single square topped impulse. Such a solution is disclosed in U. S. Patent No. 2,420,309 to Goodall issued May 13, 1947, and entitled Impulse Generator. In that patent each of the network sections is a conventional arrangement of lumped impedances utilizing a series connected helical coil, with condensers connected between tapped points on the coil and the remaining side of the line.

While a network utilizing lumped impedance elements is satisfactory for some uses, the shape of the pulse realizable with such arrangements departs undesirably from the ideal square form which is needed for more exacting applications. This is due to the impossibility of keeping the reflection coeflicients at the junction of the sections of the line real and constant functions of frequency. This is especially true at the higher impedance portions of the line.

It is an object of this invention to provide a transmission line construction, the characteristic impedance of which remains constant over a wide range of frequencies.

It is a further object of the invention to provide a transmission line construction, which when utilized in a multiple line network will produce a more nearly ideal square topped pulse than could be achieved by the use of previously known lines.

It is another object of the invention to provide a transmission line construction which will, at higher power outputs, have a higher discharge efliciency for a given volume than previously known lines.

It is still another object of the invention to provide a transmission line construction which occupies less space,

2 weighs less and is easier to manufacture than previously known constructions.

The above and other objects and advantages of the invention are realized by a transmission line construction in which a conductivetape is interleaved with insulating material and wound into a spiral coil with exposed lateral edges. The coil is sandwiched between a pair of conductive plates separated from it by insulation. While one such transmission line segment may be used as a network section for pulses of extremely short duration, for pulses of greater duration and better waveform segments are formed using three coils in side-by-side relationship with intervening conductive plates. The sense of the winding of the central coil is reversed with respect to that of the outer coils and the coils are connected in parallel.

In the drawing: 7

Fig. 1 is a block diagram of a typical multiple line pulse forming network; I

Fig. 2 is an exploded elevational view of the elements of a line segment utilizing one coil;

Fig. 3 is a schematic representation of a line segment utilizing a single coil;

Fig. 4 is a schematic representation of a line segment utilizing three coils;

Fig. 5 is an exploded elevational view of the arrangement of elements in a line segment of the type shown in Fig. 4; and,

Fig. 6 is a cross-sectional view of a network section in its casing.

Referring now to Fig. l of the drawing, there is shown a multiple line pulse forming network of the type disclosed in the above mentioned patent to Goodall. The network comprises a plurality of sections having different impedance values, of which four sections 1, 2, 3 and 4 are shown. The manner of selecting the number of sections and the characteristic impedance of each are fully disclosed in the patent. The terminating section 4 is connected to a magnetron which in turn feeds an antenna 6. The line is charged from a D. C. source. 7. A switch tube 8 normally maintains open a connection between the terminals of section 1. A trigger source 9 periodically causes the switch tube to short circuit the terminals of section 1, whereupon the network discharges through the magnetron in a single square topped energy pulse.

In accordance with the invention, each of the sections of the network is an artificial line composed of one or more segments of the types illustrated in Figs. 2 to 6.

Fig. 2 shows an elementary form of the line in exploded form. The line comprises a spirally wound pie 10 composed of alternate windings of conductive tape 11 and paper or other dielectric tape 12. The paper may be impregnated with Bakelite varnish or similar material. After the pie is formed the lateral faces are machined to a smooth plane surface. Fitting against each side of the spiral pic 10 are two plates 13 of dielectric material. Fitting against the exposed sides of these plates are two conductive plates 14. All of the plates are preferably formed with central openings to match the central opening in the pie 1%). The sole purpose of the conductive plates 14 is to provide capacitance to the edges of the coil turns. By this means a truly distributed inductance and capacitance are obtained.

The manner of connecting the elements of the segment is shown in the schematic representation of Fig. 3. The plates 14 are connected together by conductors 15 which are preferably constituted by an enclosing can. The conductors 15 provide an input terminal 16 and an output terminal 16 which are usually grounded. The outside terminal 17 of the coil 10 acts as one of the output terminals while the inside terminal 18 serves as an input terminal.

A network employing transmission line segments of the elementary form in Figs. 2 and 3 will satisfactorily generate pulses of extremely short duration when the segments are employed singly as network sections. When several such segments are joined to form a network section, pulses of longer duration may be generated. Networks formed of such sections, however, have a somewhat distorted output due to coupling between the segments.

Coupling can be markedly reduced by the use of a segment of the type shown in Fig. 4, which is the preferred form of the invention. This segment uses three of the coils 10 each sandwiched as before between dielectric plates 13 and conductive plates 14. In this form, it will be noted that the adjacent coils 10 are spiralled in opposite directions. The coils are connected in parallel between the terminals 17 and 18 and the plates 14 are connected as before.

An exploded view of the arrangement of coils and plates in a line as schematically illustrated in Fig. 4 is shown in Fig. 5. The arrangement of the plates 13 and 14 is the same as before, the direction of winding of the tape of the middle coil 10 being reversed with respect to that in the outer coils 10.

Fig. 6 shows two line segments of the type of Figs. 4 and connected to form a network section and housed in a container 20 in a conductive cylinder. The inside terminals of the three coils to the left of the figure are connected to the terminal 18. The outside terminals of all the coils are connected to a conductor 21. The inside terminals of the three coils on the right of the figure coils are connected to the terminal 17. The container is in contact with the peripheries of the plates 14 and the terminals 16 and 16' are connected to it. It may be grounded as shown.

The design of a network using line segments in accordance with the invention may be accomplished with the aid of empirically derived formulae by the following steps:

(1) Calculate the section impedances from:

where:

e is the dielectric constant.

D is the radio of copper tape thickness to insulating tape thickness in the coil. D can be chosen to make S a convenient integral number.

(4) Find the rth section turns per unit radial length from:

l2 N,=[ /D 5.4. (5) Compute the tape width (W,) for the desired efiiciency from:

where:

W =the sum of the 3 coil tape widths in a 3 coil segment. r is the section sequence number.

( 3 is the ratio of energy to be dissipated in rth section of network to the energy dissipated in the load.

=resistivity of the conductive tape. t thickness of the conductive tape.

(6) S is corrected from the measured segment pulse length.

(7) Z is adjusted by varying t so that:

where 1- is the delay time of the section and C is the capacitance of the rth section.

While transmission lines embodying the invention have been shown only in connection with shunt discharging pulse forming systems, it should be realized that they may also be satisfactorily employed in the classical series discharging pulse generating systems.

What is claimed is:

1. A three terminal transmission line comprising three spirally wound coils of conductive tape, the turns of each of said coils being insulated from each other, insulating material covering each of said coils, said coils being assembled in side-by-side coaxial relation with the sense of the winding of the central coil reversed with respect to that of the outside coils, a conductive plate positioned between said central coil and each of said outside coils and covering the lateral surfaces thereof, a conductive plate juxtaposed to and covering the free lateral surface of each of said outside coils, said coils, material and plates being compactly arranged, means conductively connecting said plates, means conductively connecting the inner terminals of said coils, and means conductively connecting the outer terminals of said coils, the terminals of said line being constituted by said means conductively connecting said plates, and said inner and outer terminals of said coils.

2. A three terminal transmission line comprising a plurality of serially connected transmission line segments, each of said segments comprising three spirally wound coils of conductive tape, the turns of each of said coils being insulated from each other, insulating material covering each of said coils, said coils being assembled in sideby-side coaxial relation with the sense of the winding of the central coil reversed with respect to that of the outside coils, a conductive plate positioned between said centrol coil and each of the outside coils and covering the lateral surfaces thereof, means conductively connecting the inner terminals of said coils to form a terminal of said segment, means conductively connecting the outer terminals of said coils to form another terminal of said segment, conductive container means confining said segments and the elements of said segments in close side-by-side relationship, and a conductive plate positioned between each of said segments and the segments juxtaposed thereto, said container conductively connecting all of said plates, the terminals of said line being constituted by said container, and the free terminals of the coils of the first and last segments of said series.

References Cited in the file of this patent UNITED STATES PATENTS 1,846,801 Finnegan Feb. 23, 1932 1,934,722 Lesh Nov. 14, 1933 2,000,441 Given May 7, 1935 2,362,470 De Rosa Nov. 14, 1944 2,440,652 Beverly Apr. 27, 1948 2,462,410 Lindenblad Feb. 22, 1949 2,521,963 Beusman Sept. 12, 1950 2,524,754 Bjorklund Oct. 10, 1950 2,598,683 Golay June 3, 1952 2,626,317 Malm Jan. 20, 1953 FOREIGN PATENTS 632,834 Great Britain Dec. 5, 1949 

